1. Field of the Invention
The present invention relates to a screen cylinder intended for use in screening wood pulp and other fibrous fluid suspensions for removing foreign particles from a pulp slurry, and a method for manufacturing the apparatus. More particularly, the apparatus and method relate to rebuildable screen cylinders for screening thick stock or thick pulp slurry within the pulp and paper industry. Still more particularly, the apparatus and method relate to improved screen cylinders that are less expensive to manufacture and provide increased wear life and durability as compared to similar screen baskets known heretofore.
2. Description of the Prior Art
Screens are used to separate acceptable papermaking fiber from unacceptable constituents of a slurry of pulp fiber in preparing the wood fiber for the papermaking process. In typical wood pulp screens, a slurry of pulp flows through a perforate cylindrical screen plate or basket which may be smooth, or which may have a contoured surface facing toward the stock flowing through the screen cylinder. The screen plate openings are formed in different hole or hole and slot combinations for optimizing screening performance. To aid in passage of the acceptable pulp through the screen plate, and to avoid plugging, pulsations are generated in the slurry such as by passing a hydrofoil-shaped member or rotor past the screen plate. Screen cylinders used in pulp and paper mills are subjected to heavy pressure loads. To provide sufficient strength to a screen cylinder or a screen plate, which generally is a basket-shaped member surrounding a rotor, so that it can withstand the pressures experienced in a pressurized screen cylinder, it has been the accepted practice to provide thick-walled screen plates or baskets which are machined to present the desired screening surface, or thin-walled formed screen plates or baskets with reinforcing rods.
A thick-walled screen cylinder is described in U.S. Pat. No. 3,664,502. Screen cylinders of the type described in this patent are formed of a metal plate rolled and welded in tubular form and provided with a multiplicity of screening openings. To withstand the pressures involved, relatively heavy gauge metal is used, such as 1/8" to 5/16" stainless steel. When the screen plate of the type described in this patent is rolled into tubular form, a weld seam is used to connect the ends of the metal plate. In order to perform the welding operation, a welding fixture must be utilized to hold the rolled screen plate in proper position to perform the weld. The weld seam leaves a rough, abrasive surface on the inside diameter of the rolled screen plate. Left as is, the weld seam would drastically affect the screening surface of the screen plate and reduce the effectiveness and efficiency of the screening operations. As a result, screens of this type that are rolled and welded into tubular form must undergo grinding operations to smooth out the interior surface of the screen plate. After the grinding operations, the screen cylinder is machined in the round or rolled condition to provide a finished interior surface.
In addition to the expensive costs of production and manufacturing, in large part due to the lengthy welding and grinding procedures, the type of one-piece screen described above has been expensive to use and maintain in that, even if only a small area of screen is damaged, the entire screen plate, which includes the screening surface, mounting bodies and support members must be replaced, thereby presenting a costly operating experience.
An improvement to the solid, one-piece, thick-walled screen is described in U.S. Pat. No. 4,264,438. The screen cylinder or drum according to this patent is assembled into cylindrical form by using a plurality of adjacent cylindrical screen members spaced apart, between which a stiffening ring is disposed. The cylindrical screen members and the stiffening ring are attached to each other by a weld joint connecting a projecting part of the stiffening ring and the ends of the cylindrical screen members and filling the gap between them. As with the prior art screen described in U.S. Pat. No. 3,664,502, because of the welding operations needed to connect the ends of the screen members and the screen members to the rings, welding fixtures are necessary to hold the screen assembly in proper position. The numerous weld seams must be ground smooth on the interior surface of the screen so as not to disturb the screening operations, and finish machining is also required. The lengthy welding and grinding operations to this prior art screen cause slot and hole distortion in the heat affected zones of the screening media. Because of the welding stresses that occur in the welds connecting the cylindrical screen members and the stiffening rings, the teaching of the patent for this prior art screen provides that the ends of the cylindrical screen members should be expanded before welding by the same amount as they are contracted by the welding stresses. As a result, manufacturing a screen drum according to the method described in U.S. Pat. No. 4,264,438, is extremely costly and time consuming.
The prior art screens described above require that the screen cylinder ends be seam welded when rolled into cylindrical form. This manufacturing method of construction leads to screen failure at the welded seam when the screen is used under normal operating conditions. The welded seam joint constrains the cylinder screen in the round condition under mechanical stress, and the welding process induces thermal stresses in the screen at locations near the weld seam. The weld seam creates a heat-affected zone at and near the seam which becomes very brittle. Thus, under normal operating conditions, the screen is subject to failure at or near the weld seam. To help overcome these problems, stress relieving is performed in one of two ways to prevent or reduce the stresses introduced into these prior art screens. The first method involves vibratory stress conditioning of the screen, and the second method involves thermal stress relieving the screen by heat treatment. However, internal stresses of the nature created in manufacturing these prior art screens are not always successfully stress relieved by the above methods, and, as a result, the potential for failure is not eliminated; and it has been observed in prior art screen cylinders that fractures tend to occur along the welded seam and heat-affected zones even under normal operating conditions.
U.S. Pat. No. 4,954,249 describes an improved screen over the screens described above as used in the pulp and paper industry. Beloit Corporation sells and markets screen cylinders according to this patent under the trademark BelWave.TM.. The modular screen plate structure of Beloit's BelWave.TM. screen simplifies screen plate changing and eliminates the need to change an entire screen plate when only a portion of the plate is damaged or worn. One of the features of Beloit's BelWave.TM. screen plate is utilizing corrugated, thin-walled screen material in order to avoid the attendant difficulties of machining thick-walled screen plates and to reduce the cost associated with manufacturing thick-walled screen plates. The modular, cylindrically-shaped screens also reduce the number of welding operations needed to create cylindrical screens by positioning and connecting modular screen sections into grooves located in support rings. The modular screen sections are formed into a corrugated pattern and then rolled into cylindrical form. One end of the corrugated screen plate section overlaps the other end of the corrugated section and a weld seam is not required to hold the ends together because the corrugated thin-walled section is pressed into the grooves located in adjacent support rings.
Although Beloit's BelWave.TM. screen cylinder has been and continues to be an improved screen plate for the pulp and paper industry, in certain thick stock or slurry screening operations, the thin, corrugated screening media is subject to impact failure.
What is needed is a screen cylinder that utilizes the benefits of Beloit's Modular BelWave.TM. screen cylinder construction and yet is capable of withstanding the high pressure and wear due to contaminants encountered in thick slurry environments and, at the same time, eliminate the disadvantages and problems associated with manufacturing screens of the types described above.